Downhole gas detection in drilling muds

Apparatus and method are disclosed for detecting gas in return fluids. A semi-permeable interface is disposed on a tool sub, the semi-permeable interface being in communication with a fluid cell and with a return fluid in an annular region defined by an outside the tool sub and a borehole wall. A sensor responsive to a gas carried by the return fluid is disposed in the fluid cell to sense the gas that enters the fluid cell from the return fluid via the semi-permeable interface.